When a rotor for a steam turbine is manufactured, the rotor material is selected from the viewpoint of high-temperature strength to temperature distribution within the steam turbines. Specifically, as the rotor material, 10% Cr steel which is strengthened by the inclusion of 0.02% or higher of nitrogen, 10% Cr steel (high Cr steel) containing W (tungsten), or the like is used in a high temperature region exceeding 566° C., 1 to 2.25% CrMoV low alloy steel is used in an intermediate region of 566 to 380° C., and 3.5% NiCrMoV low alloy steel is used in a low temperature region of under 380° C. Additionally, in an environment where the high temperature region, and the intermediate and low temperature regions exist together, an integral turbine rotor made of high Cr steel which has a strength corresponding to the high temperature region is used.
However, since the high Cr steel is a high-cost material, it is a great burden in terms of cost to form the whole turbine rotor, used in an environment where the high temperature region and the intermediate and low temperature regions exist together, from the high Cr steel.
Thus, a dissimilar steel welded rotor is suggested which is used for a steam turbine in which the high temperature region and the intermediate and low temperature regions exist together within the steam turbine, and in which a turbine rotor part which is arranged in a position where the environmental temperature within the steam turbine is the intermediate or low temperature regions is formed from an inexpensive low alloy steel, and a turbine rotor part which is arranged in a position where the environmental temperature within the steam turbine is the high temperature region is formed from high Cr steel having excellent high-temperature strength.
As the dissimilar steel welded rotor, for example, a turbine rotor in which a high Cr steel rotor and a low Cr steel rotor are welded together using a filler material in which the content of Cr steel is 1.0 to 3.5% by mass % is disclosed in Patent Document 1.
However, in the conventional turbine rotor as disclosed in Patent Document 1, in which a high Cr steel rotor member and a low Cr steel rotor member are welded together, generally, 0.02% or higher of nitrogen is contained in high Cr steel by mass % in order to increase its high-temperature strength. However, when a 10% Cr steel or a 10% Cr steel containing W, and low Cr steel are welded together, there is a problem that minute blowholes are apt to be generated in the filler material by the nitrogen contained in the high Cr steel.
Especially when a filler material of low Cr steel is used, since the content of Cr in the filler material is small, it is confirmed that the solubility of the nitrogen in the filler material is low, and minute blowholes are apt to be generated. Since the turbine rotor is a high-speed rotating body, a rotor defect that undergoes repeated fatigue is not permitted, and even minute blowholes become a problem if the blowholes are concentrated, so a secure welded joint with no blowholes is required.
Additionally, when a rotor member for high temperature made of 12% Cr steel (and broadly, 10% Cr steel) and a rotor member for low temperature made of low alloy steel are welded together to manufacture a dissimilar steel rotor, Patent Document 2 discloses a technique of manufacturing a dissimilar steel rotor through a step of performing build-up welding by a suitable material on the high temperature rotor member to form an intermediate member, performing high-temperature heat treatment on the high temperature rotor member having the intermediate member, a step of welding the low temperature rotor member to the intermediate member, and a step of performing low-temperature heat treatment on both the high temperature rotor member and the low temperature rotor member.
However, since build-up welding and two heat treatments are required in the technique disclosed in Patent Document 2, soaring manufacturing costs and prolonged manufacturing time become problems.
Additionally, Patent Document 3 discloses a technique of manufacturing a dissimilar steel rotor by forming the high temperature rotor member from 9% Cr steel (and broadly, 10% Cr steel), and forming the low temperature rotor member from 1% Cr steel, welding together the high temperature rotor member and the low temperature rotor member by using a 9% Cr-based filler material, and performing heat treatment on the welded member at 625 to 655° C. However, minute blowholes may be generated even in the technique disclosed in Patent Document 3.
That is, in the technique disclosed in Patent Document 1, in a case where the high Cr steel rotor member, which contains 0.02% or higher of nitrogen by mass % in order to increase high-temperature strength, and the low Cr steel rotor member are directly welded together, there is a problem in that minute blowholes may be generated in the filler material due to the nitrogen contained in the high Cr steel. In addition, if not low Cr but high Cr steel, such as 9Cr steel, is used as the filler material used for the welding, blowholes are hardly generated. However, since generation of the blowholes is not completely eliminated, the problem cannot be solved.
Additionally, since build-up welding and two heat treatments are required in the technique disclosed in Patent Document 2, there are problems in terms of manufacturing costs and manufacturing time.
Additionally, in the technique disclosed in Patent Document 3, even in the case of a high intermediate pressure turbine in which a dissimilar steel rotor using 9% Cr steel and 1% Cr steel is used in the high and intermediate temperature regions, a problem that minute blowholes may be generated occurs.